Introduction
Pulmonary arteriovenous malformation is a rare congenital disorder. It causes an abnormal connection between the pulmonary artery and vein. It is a vascular disease that consists of feeding arteries, nidus, and draining veins. It is also referred to as pulmonary arteriovenous aneurysms, cavernous hemangiomas, arteriovenous angiomatosis, pulmonary hamartomas, and pulmonary angiomas.
What Is Pulmonary Arteriovenous Malformation?
The pulmonary arteries carry blood from the heart to smaller arteries and then to smaller vessels known as capillaries. Capillaries are present within the lungs to slow down the blood flow and allow nutrient and gas exchange between blood and the lungs. Pulmonary veins carry blood away from capillaries and back to the heart. Pulmonary arteriovenous malformation refers to the abnormal connection between the pulmonary artery and vein that bypasses the pulmonary capillary bed.
This results in a right-to-left shunt, reduced gas exchange, and decreased oxygen levels. The arteries and veins have weak walls in arteriovenous malformations, resulting in lung bleeding. Blood clots or bacteria in the circulation can get trapped in the capillaries and pass through the malformation to reach the brain's blood vessels causing stroke or brain abscess. The pulmonary arteriovenous malformation can also cause an abnormal connection between the bronchial artery and the pulmonary vein.
What Are the Causes of Pulmonary Arteriovenous Malformation?
The exact cause of pulmonary arteriovenous malformation is not known. It can be a congenital or acquired disorder. The congenital condition is associated with the autosomal dominant disorder, hereditary hemorrhagic telangiectasia (HHT), also called Rendu-Osler-Weber disorder. The acquired condition associated with a pulmonary arteriovenous malformation is liver cirrhosis. In individuals with liver cirrhosis, the absence of hepatic factors may result in pulmonary arteriovenous malformation.
The congenital heart disease in which the systemic venous return to the lungs that does not include blood return from the hepatic veins can result in pulmonary arteriovenous malformations. It can also occur secondary to chronic infections like schistosomiasis, actinomycosis, tuberculosis, and metastatic thyroid cancer. Fanconi syndrome and mitral stenosis are also associated with this malformation.
What Are the Effects of Pulmonary Arteriovenous Malformation?
The pulmonary capillary bed is responsible for oxygen exchange. The blood bypasses the pulmonary capillary bed in pulmonary arteriovenous malformation, and returns desaturated blood to the pulmonary veins. This leads to arterial oxygen desaturation and cyanosis (bluish skin discoloration). The arteriovenous malformations are usually found in the lower lobes of the lungs. This causes orthodeoxia (fall in arterial oxygen saturation) in an upright position because gravity directs more blood to the lower lobes. Hematocrit and hemoglobin concentrations increase in cyanosis but bleeding from epistaxis, and gastrointestinal telangiectasias reduce the hemoglobin concentrations and result in iron deficiency anemia. Large pulmonary arteriovenous malformations allow blood clots from other sites to enter the left atrium and systemic arterial circulation, resulting in a stroke.
How Does Pulmonary Arteriovenous Malformation Occur?
The terminal arterial loops with a defect allow dilatation of the thin-walled capillary sacs. The incomplete resorption of the vascular septa that separate the arterial and venous plexus causes pulmonary arteriovenous malformation. This normally causes anastomosis during fetal development. The arteriovenous malformation can also occur due to failure in capillary development during fetal growth. The progressive dilation of the smaller plexus leads to the formation of tortuous loops and multiloculated sacs, which results in the development of large saccular pulmonary arteriovenous malformation. The intervening vascular walls may rupture and result in the formation of a single large saccular malformation.
What Are the Types of Pulmonary Arteriovenous Malformation?
Pulmonary arteriovenous malformations are classified into simple, complex, and diffuse types.
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Simple Type - It is the usual type of pulmonary arteriovenous malformation. It presents with a single segmental artery that feeds the arteriovenous malformation. The feeding artery may have multiple subsegmental branches.
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Complex Type - This malformation has multiple segmental arteries that feed the malformation.
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Diffuse Type - It is a rare malformation that presents hundreds of malformations. It can also occur as a combination of simple and complex types.
What Are the Symptoms Associated With Pulmonary Arteriovenous Malformation?
Individuals with pulmonary arteriovenous malformations usually do not present any symptoms. If symptoms occur, it includes the following:
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Difficulty in breathing.
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Difficulty in exercising.
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Sputum with blood.
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Nose bleeding.
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Chest pain.
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Cyanosis (bluish discoloration of the skin).
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Clubbing of finger
What Are the Complications Associated With Pulmonary Arteriovenous Malformation?
The complications of pulmonary arteriovenous malformation include:
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High output cardiac failure.
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Polycythemia.
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Paradoxical cerebral embolism.
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Cerebral abscess.
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Pulmonary hypertension.
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Hypoxemia.
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Orthodeoxia.
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Hemothorax.
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Infectious endocarditis.
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Severe hemoptysis.
How Is Pulmonary Arteriovenous Malformation Diagnosed?
The diagnosis is based on the following investigations:
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Echocardiogram - This is used to rule out other causes of intracardiac right-to-left shunt.
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Echocardiogram With Bubble Contrast - A solution of 8 ml of saline is mixed with 1 ml of the patient's blood, and 1 ml (milliliter) of air is agitated to produce microbubbles and then injected through a peripheral intravenous catheter. This is used to image the heart. The microbubbles cause a bright echo reflection. The microbubbles pass through the arteriovenous malformation and are visible in the left atrium and ventricle in the field of view.
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Chest Radiographs - Chest radiographs reveal the presence of large malformations and associated structures.
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CT (Computed Tomography) Scan is used to visualize abnormal vascular structures.
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Radionuclide Perfusion Lung Scan - Technetium 99m is intravenously injected to view the arteriovenous malformation.
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Magnetic Resonance Imaging (MRI) Scan - It is used to diagnose the malformation.
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Pulmonary Angiography - Contrast-enhanced angiography is the standard criterion to diagnose unsuspected arteriovenous malformations.
How Is Pulmonary Arteriovenous Malformation Treated?
Embolization therapy is the mainstay treatment for pulmonary arteriovenous malformation. It is performed to occlude the feeding arteries to pulmonary arteriovenous malformations. The malformations are located using angiography, and the feeding artery is selectively catheterized. A steel coil is advanced through the catheter to occlude the vessel. A balloon catheter can also be used to occlude the vessel. Surgical resection is performed in patients with an untreatable allergy to contrast material. Local excision, ligation, segmentectomy, lobectomy, or pneumonectomy were performed. However, embolization is the treatment of choice because major surgery, general anesthesia, and loss of pulmonary parenchyma can be avoided.
Conclusion
Pulmonary arteriovenous malformation is a rare disorder. It can be treated. Individuals with pulmonary arteriovenous malformations are usually asymptomatic. Early diagnosis and treatment can prevent the risk of complications.
